1. Field of the Invention
The invention relates in general to an image sensing device and more particularly to an image sensing device used for scanning a document.
2. Description of the Related Art
Referring to FIG. 1, a schematic diagram of an optical path shows a sensing unit module 118 senses the image of a document 112. The sensing unit module 118 has a long strip shape, and includes 3 sensing units, R, G, and B. Each sensing unit is capable of sensing a scan line 122 on the scan document 112 in an X-axis direction. The resolution in the X-axis direction is referred as “optical resolution”, and dpi represents a dot number of light sensing elements covered by each sensing unit projected from every inch of document. Moreover, since a machine or a scan document 112 is driven by a step motor to transmit in a Y-axis direction, a sensing unit module 118 can sense a plurality of scan lines 122 on the scan document 112. The resolution in the Y-axis direction is referred as “mechanical resolution”, which represents step number by which the sensing unit module 118 is transmitted per inch of the step motor. Therefore, the resolution of the scanner is normally represented by (X-axis resolution×Y-axis resolution) dpi, such as 600×300 dpi.
After the 3 sensing units R, G, and B sense 3 adjacent scan lines Ln, Ln+1 and Ln+2 respectively through a lens set 116, the step motor drives the sensing unit module 118 to the next step along the Y-axis (dotted lines of FIG. 1), the sensing units exactly sense next 3 adjacent scan lines Ln+1, Ln+2 and Ln+3. Therefore, the image of each scan line 122 is sensed by corresponding sensing units R, G, and B, and a complete color image can thus be obtained.
Let the step motor used in a scanner whose Y-axis resolution is 300 dpi be taken for example. When the Y-axis resolution is increased to 600 dpi, in order to maintain the same scan speed, a number of scan lines per page needs to be doubled, making the scan time per scan line halved. Since the exposure time of each sensing unit is halved, the exposure capacity will become insufficient.